Three Israelis, one of whom is 81 years old, have been convicted of running one of the biggest cannabis-smuggling operations ever intercepted by British police.

Grandfather Moshe Kedar, Yehezkel Srebro, 57, and 67-year-old Mordechai Hersh, were behind an elaborate plot to bring more than 12 tonnes of the drug, with a street value of £36 million, into the country using a tugboat.

They were arrested in April last year, with nine other men, following extensive surveillance work by teams from the Serious Organised Crime Agency (SOCA) and customs officers working on intelligence tip-offs from Israel.

The three Israelis were convicted of drug smuggling on Tuesday following a 15-week trial at Winchester Crown Court. Srebro pleaded guilty before the trial, but the other two had denied their involvement. They were due to be sentenced yesterday.

More than 400 bales of Moroccan cannabis resin were concealed on the boat. It took officers five days to remove the haul.

Andy Sellers, SOCA deputy director, said: “This operation was a massive success for international and national law enforcement co-operation. The quantity of drugs seized would have funded a wide range of other criminal activity.”

SOCA began following the gang in January 2008 when Israeli detectives contacted them after intercepting the men’s calls.

The tug docked in Southampton in April last year after sailing from Ceuta, on the northern tip of Morocco, a month earlier.

Kedar was the logistics man behind the operation. A career in construction and extensive marine knowledge made him the perfect candidate to source and manage the tug boat, SOCA investigators said.

After the gang bought the vessel in 2006, Kedar was responsible for organising extensive repairs.

Despite his age and more than a year on remand in jail, he is thought to be in good health. A leading officer in the case described him as “friendly” and said he had been “amusing in the witness box” during the trial.

On one of Kedar’s visits to Britain while planning the operation, surveillance officers watched as he stopped his hire car in the middle of a dual carriageway to read a map.

After his arrest, he claimed not to have known the boat was being used for drug smuggling and said he thought it was being brought to Britain for tug work.